Protecting medium voltage inductive coupled device from electrical transients

ABSTRACT

There is provided a method for protecting loads associated with power distribution system inductive signal couplers. The method includes (a) providing an inductive signal coupler having a first winding in series with a line conductor of a power distribution system, and a second winding having first and second connection terminals, (b) connecting a first terminal of a first fuse to the first connection terminal, and a first terminal of a second fuse to the second connection terminal, a second terminal of each fuse being connected to a communication device, and (c) connecting a first terminal of a first choke to the second terminal of the first fuse, and a first terminal of a second choke to the second of the second fuse, a second terminal of each choke being connected to an electrical ground.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present application is claiming priority of U.S. ProvisionalPatent Application Serial No. 60/364,321, filed on Mar. 14, 2002, andU.S. Provisional Patent Application Serial No. 60/376,377, filed on Apr.29, 2002.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to coupling communication signalsto electrical power distribution systems.

[0004] 2. Background of the Related Art

[0005] Radio frequency (rf) modulated data signals can be coupled to andcommunicated over medium and low voltage power distribution networks.Use of inductive couplers for this purpose is described in U.S. Pat. No.6,452,482, entitled “Inductive Coupling of a Data Signal to a PowerTransmission Cable”, and U.S. patent application Ser. No. 10/082,063,filed Feb. 25, 2002, entitled, “Coupling Broadband Modems to PowerLines”, both of which are assigned to the assignee of the presentapplication, and the contents of which are incorporated herein byreference.

[0006] Power distribution networks are occasionally subject tosignificant transients in voltage and current. For example, a strongcurrent pulse of fast rise time is created when a power line device suchas a distribution transformer short circuits, or when power lines falland touch each other. Similarly, a lightning strike to a nearby point onthe power line generates a traveling wave on the power line. A standardmethod of simulating a lightning strike is the Basic Impulse Loading(BIL) pulse, used for testing power line devices that would be connectedto power lines, and it has a rise time of 1.2 microseconds, with a muchlonger fall time. The amplitude of such test pulses can vary between 90and 200 kV peak.

[0007] A power line inductive coupler is basically a transformer whoseprimary is connected to the power line and whose secondary is connectedto a communications apparatus such as a modem. The primary winding hasone or just a few turns and presents a very low impedance at a powerfrequency. However, the coupler is capable of coupling the highfrequency energy represented by the fast onset of a lightning pulse orother transient, and substantial voltage would be induced in the couplersecondary circuit.

[0008] Coupler flashover of medium voltage from a primary power wire toground occurs when the wire's voltage exceeds the insulation capabilityof the coupler, whether during normal operation or during transientvoltage pulses originating in lightning strikes or switching transients.Flashover can occur on the outer surface of the coupler or internallybetween parts of the coupler. Flashover may be considered a very rareevent for suitably insulated devices attached to a medium voltage powerline. For example, current and potential transformers commonly used byutilities often do not carry special protective circuitry. But in thecase of a data coupler, which is intended to be used ubiquitously for alarge customer base, it is considered prudent to protect against rareevents, to prevent injury or damage.

[0009] In addition, since the modem is connected to lines leading tocustomer equipment, the modem is grounded. Therefore, the distributionpower voltage must be insulated from the modem. If the inductivecoupler's secondary were insulated from ground, then the voltagedifference between the power line and ground would be divided across (a)the coupler's primary to secondary insulation and (b) the insulation ofother devices in the chain of devices leading to the modem. The voltagedrops would be proportional to the impedances across each insulationinterface, and thus inversely proportional to the stray capacitanceacross each such interface.

[0010] When dealing with medium voltage ac power lines, with voltages inexcess of 2,000 volts rms relative to neutral or ground, this capacitivevoltage division would be difficult to make deterministic, as thecoupler capacitance would depend on the position and diameter of thepower line within the coupler. Therefore, any other insulating deviceswould need to be capable of insulating the full power line voltage, andthus be large and expensive.

SUMMARY OF THE INVENTION

[0011] Embodiments of the present invention are directed to techniquesfor protecting an inductive coupler of data signals to a powerdistribution network from electrical transients such as transientover-voltage and over-current conditions. More specifically, embodimentsof the present invention enable an inductive coupler to withstandvoltage spikes and provides protection against surges from flashover,i.e., the sudden breakdown of electrical insulation in the coupler, withoptimal coupling of the rf data signal between an rf data modem and thepower line. Embodiments also protect against transient current pulsesthat may develop on the power distribution line from such causes as alightning strike or short-circuiting of the line to electrical ground.

[0012] A method for protecting loads associated with power distributionsystem inductive signal couplers includes (a) providing an inductivesignal coupler having a first winding in series with a line conductor ofa power distribution system, and a second winding having first andsecond connection terminals, (b) connecting a first terminal of a firstfuse to the first connection terminal, and a first terminal of a secondfuse to the second connection terminal, a second terminal of each fusebeing connected to a communication device, and (c) connecting a firstterminal of a first choke to the second terminal of the first fuse, anda first terminal of a second choke to the second of the second fuse, asecond terminal of each choke being connected to an electrical ground.

[0013] Another method for protecting loads associated with powerdistribution system inductive signal couplers includes (a) providing aninductive signal coupler having a first winding in series with a lineconductor of a power distribution system, and a second winding havingfirst and second connection terminals, (b) encapsulating the secondwinding inside a layer of electrical insulation, and (c) connecting thesecond winding to an electrical ground using protection circuits so asto place any high voltage field across the layer of electricalinsulation.

[0014] Another method for protecting loads associated with powerdistribution system inductive signal couplers includes providing aninductive signal coupler having a first winding in series with a lineconductor of a power distribution system, and a second winding havingfirst and second connection terminals, in which the coupler has a bodyincluding sheds providing a leakage path to avoid external flashoverduring an electrical transient.

[0015] Another method for protecting loads associated with powerdistribution system inductive signal couplers includes (a) providing aninductive signal coupler having a first winding in series with a lineconductor of a power distribution system, and a second winding havingfirst and second connection terminals, the coupler having a bodyincluding a conductive plate at an end of the coupler distal from thefirst winding, and (b) connecting the conductive plate to an electricalground so as to route a flashover current directly to the electricalground.

[0016] Another method for protecting loads associated with powerdistribution system inductive signal couplers includes (a) providing aninductive signal coupler having a first winding in series with a lineconductor of a power distribution system, and a second winding havingfirst and second connection terminals, and (b) connecting each terminalof the second winding to an electrical ground via a choke, the chokepresenting a high impedance to signal frequencies and a low impedance tocurrent from an electrical fault signal.

[0017] An arrangement of components includes (a) an inductive signalcoupler having a first winding in series with a line conductor of apower distribution system, and a second winding having a firstconnection terminal and a second connection terminal, (b) a first fusehaving a first terminal connected to the first connection terminal, anda second terminal for coupling a signal to a first terminal of acommunication device, (c) a second fuse having a first terminalconnected to the second connection terminal, and a second terminal forcoupling a signal to a second terminal of the communication device, (d)a first choke having a first terminal connected to the second terminalof the first fuse, and a second terminal connected to an electricalground, and (e) a second choke having a first terminal connected to thesecond terminal of the second fuse, and a second terminal connected tothe electrical ground.

[0018] Another arrangement of components includes (a) an inductivesignal coupler having a first winding in series with a line conductor ofa power distribution system, and a second winding encapsulated inside alayer of electrical insulation, and (b) a circuit between the secondwinding and an electrical ground for placing a high voltage field acrossthe layer of electrical insulation.

[0019] Another arrangement of components includes (a) an inductivesignal coupler having a first winding in series with a line conductor ofa power distribution system, and a second winding having a firstconnection terminal and a second connection terminal, (b) a first chokebetween the first connection terminal and an electrical ground, and (c)a second choke between the second connection terminal and the electricalground. Each of the first choke and the second choke present a highimpedance to a signal frequency and a low impedance to current from anelectrical fault signal.

[0020] An inductive signal coupler for coupling a signal to a powerdistribution system includes a first winding in series with a lineconductor of the power distribution system, and sheds for providing aleakage path to avoid external flashover during an electrical transient.

[0021] Another inductive signal coupler for coupling a signal to a powerdistribution system includes a winding in series with a line conductorof the power distribution system, and a conductive plate at an end ofthe coupler distal from the first winding, for routing a flashovercurrent to an electrical ground.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The present invention will be more readily understood byreference to the following detailed description taken with theaccompanying drawings, in which:

[0023]FIG. 1 shows an inductive coupler circuit according to oneembodiment of the present invention, which is protected againstover-voltage transients.

[0024]FIGS. 2A, 2B and 2C show embodiments of the present invention inwhich an arrangement of capacitors and surge suppressors protect againstelectrical transients.

[0025]FIG. 3 shows the equivalent circuit for a voltage transientcreated by flashover.

[0026]FIG. 4 shows a cross-section of one particular physicalimplementation of an inductive coupler according to an embodiment of thepresent invention.

[0027]FIGS. 5A and 5B shows particular specific implementations of adual fuse according to an embodiment of the present invention.

DESCRIPTION OF THE INVENTION

[0028] One embodiment of the present invention grounds the secondarywinding of an inductive coupler by appropriate rf devices. This protectsagainst over-voltage transients and takes full advantage of theprinciple of magnetic coupling, which is not affected by the thicknessof the winding insulation. Consequently, the medium voltage of the powerline is insulated from the modem solely by the insulation of thesecondary winding. This approach prevents flashover current frompropagating to low voltage lines and loads, and so prevents damage tothe modem and other equipment to which the coupler may be connected.

[0029]FIG. 1 shows an inductive coupler circuit according to oneembodiment of the present invention, which is protected againstover-voltage transients. Power distribution line 100 forms a primarywinding 105 of inductive coupler 110, which in turn is connected to anrf data signal modem (not shown) via output terminals 160 and 165.Secondary winding 115 has terminals 120 and 125, which connect to theupper terminals of transient protection fuses 130 and 135 respectively.Rf chokes 140 and 145 connect the lower terminals of the fuses to ground150, typically via a wire 155 connected to the “pole ground,” a groundwire running from a ground rod at the base of the electric pole, up tothe top of the pole. This pole ground 150 will generally be readilyavailable in typical applications of a power line inductance coupler 110used to bypass a distribution transformer on electric power poles.

[0030] The coupler 110 physically bridges a space between the power line100 and the ground 150 connected to the coupler secondary winding 115.Thus, a leakage path is needed that is long enough to preclude externalflashover. A typical embodiment provides “sheds.” In case externalflashover occurs anyway, the coupler 110 may include an exposed metalbase connected to the ground 150 to which an external flashover arc mayjump without harm.

[0031] The rf chokes 140 and 145 are provided to ground any potentialinternal flashover current within the coupler 110. The secondary winding115 is typically embedded in insulating material body of the coupler110, which should be thick enough to provide a sufficient insulationrating for both steady state (“withstand”) voltage and for fast highvoltage BIL pulses. The rf chokes 140 and 145 provide an rf impedancesubstantially greater than the rf impedance of the coupler secondarywinding 115, while providing a low impedance to ground 150 after a fewmicroseconds of a fault pulse. The connection of chokes 140 and 145 inshunt with the signal voltage provides a high pass filtering effect, aslow frequencies are effectively shorted to ground 150. For modemfrequencies above 1 MHz, chokes 140 and 145 might typically have aninductance of 10 uH each, providing a reactance across the couplersecondary winding 115 in excess of 124 ohms and rising with frequency.The chokes 140 and 145 should have a self-resonant frequency above thehighest frequency of interest.

[0032] Flashover current is limited only by the capacity of the powerline 100, typically up to 10,000 amps rms or about 14,000 amps peak.This flashover current is interrupted by and divided roughly equallybetween the fuses 130 and 135. Until the fuses 130 and 135 blow open,the rf chokes 140 and 145 need to carry the short circuit currentwithout failing. Thus, rf chokes 140 and 145 should be wound with wirecapable of withstanding the flashover current pulse that might flow.

[0033] The speed and size of a possible flashover current pulse suggestsuse of suitably rated expulsion fuses or current limiting fuses for thetransient protection fuses 130 and 135. An expulsion fuse can interruptcurrent up to 8 milliseconds after the onset of a flashover transient. Acurrent limiting fuse may interrupt faster, estimated not to exceed 4milliseconds after the onset of a flashover transient. To maintaincompliance with electromagnetic radiation standards, data signal currentis expected to be much less than one ampere, so a 1 amp current ratingfor the fuses 130 and 135 would be suitable for minimizing the durationof any flashover current after an internal insulation failure.

[0034] Both current limiting and expulsion fuses have considerablelength and breadth, as needed to extinguish the high energy arcinitiated and maintained by the kilo-ampere short circuit current ofpower distribution lines. Placement of two such individually packagedfuses 130 and 135 next to each other creates a substantial enclosed areain the plane of the fuse pair, producing a substantial inductance inseries with the high frequency signal. It may be noticed that duringnormal operation, only the small signal voltage is applied between thefuses 130 and 135, and that during an internal flashover, they wouldboth be clearing essentially the same fault. Therefore it may beadvantageous to combine the two fuses 130 and 135 into a single housing,and share the arc extinguishing mechanism. By placing the two fuses 130and 135 in parallel with each other with a spacing and thicknesscommensurate with the characteristic impedance seen from the couplersecondary winding 115, the effect of spurious inductance and capacitancewould be minimized, to the extent that the coupler secondary impedancewas constant over frequency and known.

[0035] In the case of a current limiting fuse where wires would be woundin a double helix on a “spider” coil form, in preparation for fillingthe volume with sand, there is a further technique to reduce thespurious effects of fuse reactances. A magnetic core stick may beinserted inside the helix, transforming it into a common mode choke.Such a choke has minimal differential mode attenuation, even when thecoupling coefficient between the windings is much less than unity.

[0036] One inherent mechanism that limits the transfer of fault energyis the saturation of the coupler cores. Once a fault current causes coresaturation, magnetomotive force and induced secondary voltage arebasically clamped. Power line fault transients and surges possess awaveform containing energies over a broad spectrum of frequencies. Onlyfrequencies relevant to the modem communications should reach the modem.To that end, series capacitors can be used as high pass filters thatlimit the transient energy reaching the modem.

[0037] Another side-effect of attaching an inductive coupler to a powerline is the flow of circulation current. The inductive coupler may beviewed as a current transformer (CT), and in the choke circuitsdescribed below, the CT secondary is short circuited by the seriescombination of the two chokes.

[0038] Flashover can be treated as an instantaneous short-circuiting ofthe secondary circuit to the primary circuit, and since the chokeinductors 140 and 145 initially act as an open circuit, the entireprimary voltage would appear across each choke 140 and 145, for aninitial few tens of nanoseconds. This can be addressed as shown in FIG.2A by adding high frequency coupling capacitors 200 and surgesuppressors 205, which lower the initial instantaneous voltage impressedon both the chokes 140 and 145 and capacitors 200, by acting as atemporary short circuit for the critical first tens of nanoseconds. Thisallow use of chokes 140 and 145 and capacitors 200 whose voltage ratingis 10 to 100 times less than the peak primary voltage.

[0039] In an alternative embodiment shown in FIG. 2B, a spark gap or gastube arrestor 220 is connected across secondary winding 115, to absorbat least part of the energy coupled to the secondary by a fast risetimesurge current. The addition of this device in any of the embodimentsshown in FIGS. 1 and 2 would reduce the surge energy that subsequentsurge protectors need to safely absorb.

[0040] In an alternative embodiment shown in FIG. 2C, an additionalsurge suppressor 210 may be placed in parallel with the surgesuppressors 205. The surge suppressors 205 and 210 act as a lowimpedance when a current fault generates voltages exceeding theirclamping voltage. If the devices are identical, suppressor 210 would actas the primary voltage limiter for the differential mode, while theseries pair of suppressors 205 would act as a backup limiter in case theprimary suppressor 210 failed in the open circuit condition.

[0041] The high pass filtering of the shunt chokes and series capacitorslimits the duration of fault pulses, and allows the use of relativelylow power surge suppressors. Only such low power devices are availablewith the low terminal capacitance necessary to avoid high frequencyloading of the signal by the surge suppressors. The very smallpower-frequency impedance of a high frequency coupler reduces theelectromotive force (emf) generated in the inductor secondary 115, andthe existence of sufficient fuse resistance, or optionally the additionof a small value resistor 215 in series with each secondary lead(typically, one half to one ohm), typically can reduce the resultantcurrent flow to less than one ampere per thousand amperes flowing on thepower line 100.

[0042] We can consider an internal flashover of coupler 110, fromprimary winding 105 to the secondary winding 115, simplified here to oneterminal 120 of that winding (see FIG. 1). FIG. 3 shows the equivalentcircuit as seen by the flashover voltage transient. A 10 kV dc source300 represents the instantaneous peak voltage of a 15 kV classdistribution transformer having a typical phase to neutral voltage of7-8 kV rms. Source resistance 305 limits current to a 10 kA shortcircuit value. Transmission lines 310 and 315 represent a single phaseof overhead distribution lines. The closing of switch 320 represents aninstantaneous short circuit due to internal flashover. Resistor 325represents the resistance of a fuse such as 130 and 135, and choke 330(equivalent to choke 140 of FIG. 1) closes the circuit to pole ground335. Capacitor 340 high pass couples the communications signals to modemterminals 345, and surge suppressor 350 (equivalent to suppressor 205 inFIG. 2) protects the modem against over-voltage transients. The shuntcapacitive loading of capacitor 340 and suppressor 350 (the latteracting as nearly a short circuit during transient events) lowers theinitial transient voltage at node 355, and therefore across capacitor340, allowing use of a lower cost capacitor.

[0043]FIG. 4 illustrates in cross-section one particular physicalimplementation of an inductive coupler according to an embodiment of thepresent invention. Primary power wire 400 passes through the aperture ofmagnetic cores 405 of coupler 410. Secondary wire 415 is encapsulated inthe insulating material 417 from which the coupler 410 is molded, with athickness 420 appropriate for the line's withstand voltage and BILvoltage. Sheds 425 provide the appropriate leakage path. Conductiveplate 430 is attached to the base of the coupler body, and connected viawire 435 to the pole ground 440.

[0044] If the coupler body does not provide a sufficient leakage path orinsulation commensurate with the steady state or transient voltage onthe power line conductor 400, then a flashover could occur. Flashovercurrent external to the coupler 410 would jump to the conductive plate430, and be routed harmlessly to the pole ground 440.

[0045]FIG. 5a illustrates a dual fuse 500, as implemented in anexpulsion fuse. Wires 505 connect the fuse elements 510 to two-terminalheaders 515. The fuse elements 510 are tensed by springs 520, and theentire volume is encased in an arc-quenching material, with ports 530through which any arc gasses are expelled.

[0046]FIG. 5b illustrates a dual fuse 550, as implemented in acurrent-limiting fuse. Fuse elements 555 are wound on spider form 560and terminate on two-terminal headers 565. Optionally, the spider mayhave a hollow core in which magnetic core 570 optionally may beinserted. The entire volume is filled with sand (not shown).

[0047] Although various exemplary embodiments of the invention have beendisclosed, it should be apparent to those skilled in the art thatvarious changes and modifications can be made which will achieve some ofthe advantages of the invention without departing from the true scope ofthe invention.

What is claimed is:
 1. A method for protecting loads associated withpower distribution system inductive signal couplers, the methodcomprising: providing an inductive signal coupler having a first windingin series with a line conductor of a power distribution system, and asecond winding having first and second connection terminals; connectinga first terminal of a first fuse to said first connection terminal, anda first terminal of a second fuse to said second connection terminal, asecond terminal of each fuse being connected to a communication device;and connecting a first terminal of a first choke to said second terminalof said first fuse, and a first terminal of a second choke to saidsecond of said second fuse, a second terminal of each choke beingconnected to an electrical ground.
 2. A method according to claim 1,wherein each of said first and second chokes uses wire of sufficientgauge to withstand high current flowing during an actuation time of itsassociated fuse.
 3. A method according to claim 1, wherein each of saidfirst and second chokes possesses a self-resonant frequency in excess ofa highest frequency utilized by said communication device.
 4. A methodaccording to claim 1, further comprising connecting a first terminal ofa surge suppressor to said first connection terminal of said secondwinding and connecting a second terminal of said surge suppressor tosaid second connection terminal of said second winding.
 5. The method ofclaim 4, wherein said surge suppressor is selected from the groupconsisting of a spark gap and a gas tube surge arrestor.
 6. A methodaccording to claim 1, wherein said first fuse and said first choke forma first series resistance, and said second fuse and said second chokeform a second series resistance, and wherein said first and secondseries resistances each exceed a selected threshold resistance so as tominimize power frequency circulation current.
 7. A method according toclaim 1, further comprising: providing a first resistor in series withsaid first fuse and said first choke to form a first series resistance,and providing a second resistor in series with said second fuse and saidsecond choke to form a second series resistance, wherein said first andsecond series resistances each exceed a selected threshold resistance soas to minimize power circulation current.
 8. A method according to claim1, wherein said first and second fuses are packaged in a single commonhousing.
 9. A method according to claim 8, wherein said first and secondfuses are helical current limiting fuses having a helix axis.
 10. Amethod according to claim 9, further comprising: placing a magnetic coreis in said helix axis.
 11. A method according to claim 1, furthercomprising: connecting a capacitor in series with said second terminalof each of said first and second fuses and said communication device;and connecting a surge suppressor between a communication device side ofeach capacitor and the electrical ground, so that capacitive loadingreduces the magnitude of an initial voltage transient resulting from aflashover between said windings of said inductive signal coupler.
 12. Amethod for protecting loads associated with power distribution systeminductive signal couplers, the method comprising: providing an inductivesignal coupler having a first winding in series with a line conductor ofa power distribution system, and a second winding having first andsecond connection terminals; encapsulating said second winding inside alayer of electrical insulation; and connecting said second winding to anelectrical ground using protection circuits so as to place any highvoltage field across said layer of electrical insulation.
 13. A methodfor protecting loads associated with power distribution system inductivesignal couplers, the method comprising: providing an inductive signalcoupler having a first winding in series with a line conductor of apower distribution system, and a second winding having first and secondconnection terminals, said coupler having a body including shedsproviding a leakage path to avoid external flashover during anelectrical transient.
 14. A method for protecting loads associated withpower distribution system inductive signal couplers, the methodcomprising: providing an inductive signal coupler having a first windingin series with a line conductor of a power distribution system, and asecond winding having first and second connection terminals, saidcoupler having a body including a conductive plate at an end of saidcoupler distal from said first winding; and connecting said conductiveplate to an electrical ground so as to route a flashover currentdirectly to the electrical ground.
 15. A method for protecting loadsassociated with power distribution system inductive signal couplers, themethod comprising: providing an inductive signal coupler having a firstwinding in series with a line conductor of a power distribution system,and a second winding having first and second connection terminals; andconnecting each terminal of said second winding to an electrical groundvia a choke, said choke presenting a high impedance to signalfrequencies and a low impedance to current from an electrical faultsignal.
 16. A method according to claim 15, wherein leads of said secondwinding are connected to output terminals that are connected to acommunication device via series capacitors.
 17. A method according toclaim 16, further comprising connecting at least one low capacitancesurge suppressor across said output terminals.
 18. An arrangement ofcomponents, comprising: an inductive signal coupler having a firstwinding in series with a line conductor of a power distribution system,and a second winding having a first connection terminal and a secondconnection terminal; a first fuse having a first terminal connected tosaid first connection terminal, and a second terminal for coupling asignal to a first terminal of a communication device; a second fusehaving a first terminal connected to said second connection terminal,and a second terminal for coupling a signal to a second terminal of saidcommunication device; a first choke having a first terminal connected tosaid second terminal of said first fuse, and a second terminal connectedto an electrical ground; and a second choke having a first terminalconnected to said second terminal of said second fuse, and a secondterminal connected to the electrical ground.
 19. The arrangement ofclaim 18, further comprising a surge suppressor having a first terminalconnected to said first connection terminal of said second winding, anda second terminal connected to said second connection terminal of saidsecond winding.
 20. The arrangement of claim 19, wherein said surgesuppressor is selected from the group consisting of a spark gap and agas tube surge arrestor.
 21. The arrangement of claim 18, furthercomprising: a first resistor in series with said first fuse; and asecond resistor in series with said second fuse.
 22. The arrangement ofclaim 18, wherein said first and second fuses are packaged in a singlecommon housing.
 23. The arrangement of claim 22, wherein said first andsecond fuses are helical current limiting fuses.
 24. The arrangement ofclaim 23, wherein said helical current limiting fuses have a magneticcore.
 25. The arrangement of claim 18, further comprising: a firstcapacitor having a first terminal in series with said second terminal ofsaid first fuse, and a second terminal for said coupling said signal tosaid first terminal of said communication device; a second capacitorhaving a first terminal in series with said second terminal of saidsecond fuse, and a second terminal for said coupling said signal to saidsecond terminal of said communication device; a surge suppressor betweensaid second terminal of said first capacitor and said second terminal ofsaid second capacitor.
 26. An arrangement of components, comprising: aninductive signal coupler having a first winding in series with a lineconductor of a power distribution system, and a second windingencapsulated inside a layer of electrical insulation; and a circuitbetween said second winding and an electrical ground for placing a highvoltage field across said layer of electrical insulation.
 27. Aninductive signal coupler for coupling a signal to a power distributionsystem, comprising: a first winding in series with a line conductor ofsaid power distribution system; and sheds for providing a leakage pathto avoid external flashover during an electrical transient.
 28. Aninductive signal coupler for coupling a signal to a power distributionsystem, comprising: a winding in series with a line conductor of saidpower distribution system; and a conductive plate at an end of saidcoupler distal from said first winding, for routing a flashover currentto an electrical ground.
 29. An arrangement of components, comprising:an inductive signal coupler having a first winding in series with a lineconductor of a power distribution system, and a second winding having afirst connection terminal and a second connection terminal; a firstchoke between said first connection terminal and an electrical ground;and a second choke between said second connection terminal and saidelectrical ground, wherein each of said first choke and said secondchoke present a high impedance to a signal frequency and a low impedanceto current from an electrical fault signal.
 30. The arrangement of claim29, further comprising: a first capacitor in series with said firstconnection terminal and having a device-side terminal for coupling asignal to a first terminal of a communication device; and a secondcapacitor in series with said second connection terminal and having adevice-side terminal for coupling a signal to a second terminal of saidcommunication device.
 31. The arrangement of claim 29, furthercomprising a surge suppressor for suppressing a fault voltage betweensaid device-side terminal of said first capacitor and said device-sideterminal of said second capacitor.